Cyanoacrylate adhesives which contain a 2-cyanoacrylate as a main ingredient are widely used as instantaneous adhesives in various industrial fields such as electronic, electric and automotive fields and in leisure and household applications since they are polymerized and hardened in a short time by anionic species such as moisture on an adherent surface or in the air thanks to their high anionic polymerizability, thereby bonding various materials to each other. However, they have been disadvantageous in that they are insufficient in speed of adhesion to poorly adherent materials such as polyacetals, FRPs, chrome platings and EPDM rubbers, and porous materials such as leathers, wood and paper.
As a method for solving the above problems, it has been proposed that crown ethers as described in Patent Document 1, polyethylene glycol derivatives as described in Patent Document 2 or the like should be incorporated as hardening accelerators in 2-cyanoacrylates. Thus, adhesive compositions having an adhesion speed high enough for the above materials have been obtained.
However, the adhesive compositions which contain such hardening accelerators are disadvantageous in that they are ease to thicken and gel when left at a window exposed to sunlight or under a fluorescent lamp for a long period.
As a result of investigating the cause, the present inventors have presumed that the following phenomenon occurs. Hydroquinone is commonly used as a radical stabilizer in cyanoacrylate adhesives. When radicals are generated in the system by the action of light, hydroquinone may capture the radical to form a phenoxy radical. Since the phenoxy radical is unstable, it may abstract hydrogen from a crown ether or a polyethylene glycol. The resulting radical may react with oxygen in the system to generate a peroxide. Cleavage of the peroxide may induce radical chain reactions, thereby finally leading to radical polymerization of 2-cyanoacrylates.
On the other hand, an example using another radical stabilizer instead of hydroquinone is disclosed in Patent Document 3 in which a phenolic antioxidant is used. This avoids generation of benzoquinone which is toxic and medically undesirable, when a medical-grade 2-cyanoacrylate composition is irradiated with γ-ray for sterilization. This neither improves light stability in ordinary use, nor discloses that a hardening accelerator is used together.    Patent Document 1: Japanese Patent Laid-Open (Kokai) No. 53-129231    Patent Document 2: Japanese Patent Laid-Open (Kokai) No. 54-28342    Patent Document 3: Japanese Patent Laid-Open (Kokai) No. 7-252455